The present invention relates to a piezoelectric vibrator, and, more particularly, to one that operates in a spreading vibration mode.
A conventional piezoelectric vibrator that operates in the spreading vibration mode utilizes the spreading vibration of long or short sides of a piezoelectric element which is in the form of a rectangular plate. One example of a conventional piezoelectric vibrator is as shown in FIG. 1. In FIG. 1, reference numeral 2 designates a spreading vibration mode piezoelectric element. The piezoelectric element 2 has a vibrating plate 5 of permanently elastic metal, a piezoelectric film 8, and a vibrating electrode film 9. The centers of the short sides of the rectangular vibrating plate 5 are supported through a pair of coupling pieces 6 and 6' by a holding frame 7. The piezoelectric element 2 is accommodated in a case (not shown) by supporting the holding frame 7 therein. The piezoelectric vibrator utilizes the spreading vibration of the short sides of the vibrating plate 5. That is, the vibration of the piezoelectric vibrator is in the direction of the short sides with a node of the vibration of a line connecting the coupling pieces 6 and 6'. The frequency of the vibration relates to the length of the short sides.
In the piezoelectric vibrator as shown in FIG. 1, the dead region A (shaded with oblique lines) of the vibrating plate is formed in the vibrating plate body 5a between the coupling pieces 6 and 6'. However, in the case of a low frequency band, the effects of the dead region A on the resonance frequency characteristic of the piezoelectric element 2 can be disregarded because the vibrating plate 5 is much larger in size than the coupling pieces 6 and 6'. On the other hand, when it is required to apply the spreading vibration mode piezoelectric vibrator described above to a device having a high resonance frequency of, for example, 3.58 MHz, it is necessary to extremely miniaturize the vibrating plate 5, and accordingly the size of the coupling pieces 6 and 6' relative to the size of the plate 5 cannot be disregarded. Therefore, if the size of the piezoelectric vibrator is reduced to accommodate a high resonance frequency, then the following difficulties are caused: the resonance resistance is increased, the quality factor (Q) is decreased, and the resonance frequency characteristic and other characteristics are greatly varied, for instance. Therefore, in the prior art, in the case of such a piezoelectric vibrator for high frequency, other vibration modes such as a thickness expansion vibration mode and a thickness shear vibration mode are utilized, and it is not practical to utilize spreading vibration mode piezoelectric vibrators.